...
WHERE date_column = TO_DATE(?, 'YYYY-MM-DD')
...

my $date_str = '2015-08-08';
$sth->execute($date_str);

# OR
my $date_fmt = my $datetime_fmt = my $datetime_tz_fmt = 'YYYY-MM-DD HH24:MI:SS';
$dbh->do("alter session set nls_date_format=$date_fmt");
$dbh->do("alter session set nls_timestamp_format=$datetime_fmt");
$dbh->do("alter session set nls_timestamp_tz_format=$datetime_tz_fmt");

...
WHERE date_column = ?
...
my $date_str = '2015-08-08';
$sth->execute($date_str);

##</code><code>##

$start_date = '2015-08-01';
$end_date = '2015-09-01';
...
WHERE date_column >= ? and date_column < ?
...
$sth->execute($start_date, $end_date);